This invention relates to the production of photographic base papers and photographic print papers produced therefrom having a polyolefin film on one or both sides of a substrate. In particular, it relates to a novel method of providing adhesion between the polyolefin film and the substrate.
Polyolefin coated papers and, in particular, polyethylene coated papers, have been introduced in recent years for use as photographic base papers to which may be applied photographic emulsions to produce light sensitive photographic elements. Such elements may be used to produce photographic prints. Prior to this, polyolefins, including polyethylene, were used to coat paper for many uses, principally for use as packaging and wrapping materials. Coating of such papers was conventionally by extrusion of a molten film onto paper and running the paper coated with the hot film through the nip of a pair of press rolls. Cooling of the film was accomplished by cooling the roll in contact with the hot film. Many of the uses required a bond between the paper and the polyolefin film greater than could be obtained by simply hot pressing the polyolefin film to the paper base. The use of adhesion promoters, such as treatment of the paper with a corona discharge or chemical primers, has been described in the prior art.
U.S. Pat. No. 3,371,002 describes the use of polyesters, urethanes, or preferably a mixture of these resins as chemical primers to promote the adhesion of polyolefins to paper in the production of packaging materials. U.S. Pat. Nos. 3,411,908 and 3,501,298 utilize a treatment of the paper by a corona discharge to bond polyolefins to paper to produce photographic papers which may be treated with photographic processing solutions. U.S. Pat. No. 3,411,908 also mentioned the use of organic titanates and polyethyleneimine to improve the adhesion of polyolefins to paper. Canadian Pat. No. 795,773 suggests the use of "adhesion primers" of undisclosed composition at a size press or priming station in line with the extrusion step to produce photographic papers. The use of colloidal silica to adhere certain synthetic films to paper is known from commercial literature.
Photographic base papers which are further coated with a light sensitive layer to form photographic print papers are normally sized papers made from pulps of high chemical purity. They have a moisture barrier film on both sides of the paper to prevent penetration of photographic development chemicals into the paper thereby discoloring the paper and spoiling the resultant photographic print. Polyethylene coatings are widely used as the moisture barrier films.
One difficulty usually encountered with the use of polyethylene coatings is the lack of adhesion of the polyethylene film to the comparatively smooth heavily sized paper surface. Another problem with such papers is "edge penetration". As used herein, this is defined as a measure of the penetration of development chemicals into the edges of a photographic print paper during development thereof.
Of the known processes of bonding polyethylene to paper, only the treatment of paper by a corona discharge has had commercial acceptance where the resultant paper was to be used in contact with photographic development chemicals. This method, though acceptable, has certain disadvantages as compared to the novel process of this invention. The corona discharge process requires additional equipment and the corona treatment must be done immediately prior to extruding the polyolefin film onto the paper surface. In contrast to this, the treatment with the novel primer of this invention can be done on the papermachine using equipment normally present, and the paper from the papermachine may be stored, if desired, and subsequently extrusion coated with a polyolefin with no reduction in the strength of the bond between the paper and the polyolefin.